Coating compositions which are curable under the influence of actinic radiation, e.g. ultra violet light, electron beam and the like, are well known. These coatings, generally contain, as the main characterizing component, oligomers which are end capped with functional groups. These functional groups provide sites for actinic radiation initiated free radical polymerization. Exemplary of some of these oligomers are those which are end-capped with acrylate or methacrylate groups and which have a polyester, urethane, epoxy, polyether or acrylic backbone.
It has been found that while the oligomers used in the coating compositions contribute many useful characteristics thereto, they all suffer one serious drawback, i.e., they are too viscous to insure convenient, high fidelity application to a substrate. The oligomer viscosity can be lowered by diluting it with one or more solvents. Useful solvents are methyl ethyl ketone, ethyl acetate, xylene, toluene, acetone and the like While solvents do reduce the viscosity of the oligomer their use is not widely acceptable as the solvent must be removed from the cured coating thereby adding additional expense to the total curing process.
As an alternative to using solvents, the coating industry has turned to using reactive diluents in admixture with the oligomer. The reactive diluent reacts with the oligomer during curing and becomes a part of the cured coating. No removal is needed. Exemplary of such diluents are styrene, vinyl toluene, methyl acrylate, butyl acrylate, propyl methacrylate etc. While these diluents perform their viscosity lowering function in a manner preferred to that of solvents, they are generally quite toxic. Great care and special safety equipment are required by workers who apply coating compositions containing such diluents. In response to this toxicity problem the coating industry is now moving towards the selection of reactive diluents which are satisfactory in their viscosity reducing function and which are relatively free from known toxic effect. Such a reactive diluent is the highly touted compound, N-vinyl-pyrrolidone. This pyrrolidone, however, has a strong odor which makes it use disagreeable to those applying the coating composition.
It is therefore an object of the invention to provide an actinically curable, oligomer based, coating composition which; uses a reactive diluent, is relatively free of known toxic effect and which does not have a strong odor. It is a further object of the invention to provide a cured coating from such a composition.